1. Field of the Invention
The present invention relates to a rotary disc for a rotary encoder operative to detect a rotational angle of a rotary shaft, for example.
2. Description of the Related Art
For control of exciting currents in respective phases of a motor, an encoder is attached to a rotor shaft of the motor and employed as a means to detect a rotational angle or phase of a rotor. A rotary disc 15 for use in the encoder comprises, as shown in FIG. 7, a signal generation section 16 configured to generate a signal indicative of angular position, an angle of rotation or phase, and an attachment portion 17 adapted to be attached to the rotor shaft.
The signal generation section 16 has a pattern formed on a disc surface to generate a signal indicative of angular position, an angle of rotation or phase. An optical encoder has a certain slit pattern formed on the disc surface to generate a signal usable to detect a rotational angle/phase.
On detection of an angular position, an angle of rotation or phase of the rotor by way of the rotary disc 15 attached to the rotor shaft of the motor, an angular position, an angle of rotation or phase of the rotor is required to correspond to the position or phase of the signal generated from the signal generation section 16 in the rotary disc 15. For that purpose, parts of the attachment portion 17 are flattened to form non-rotational symmetric sections 18. These non-rotational symmetric sections 18 are configured to have a known positional relation to the positional signal generation pattern on the signal generation section 16. When attaching the rotary disc 15 to the rotor shaft, attention is paid so that the signal generated from the signal generation section 16 in the rotary disc corresponds to the angular position, angle of rotation or phase of the rotor, using the non-rotational symmetric sections 18.
FIG. 8 illustrates attachment of the rotary disc 15 to a rotor shaft 12 of a motor.
First, a DC current is supplied to a stator 10 to generate an electromagnetic attractive force to fix an angular position, angle of rotation or phase of a rotor 11. A jig 19 is employed to hold the non-rotational symmetric sections 18 of the attachment portion 17 to fit and attach the attachment portion 17 to the rotor shaft 12. In this case, a signal indicative of an angular position, angle of rotation or phase outputted from the signal generation section 16 of the rotary disc 15 can represent the angular position, angle of rotation or phase of the rotor 11 if the rotary disc is attached in a manner such that the phase of the rotary disc corresponds to that of the rotor.
The rotary disc 15 may be configured to include the signal generation section 16 and the attachment portion 17 formed integrally or separately as disclosed in Japanese Patent Application Laid-open No. H11-14404, for example.
The above-described conventional rotary disc requires a member for adjustment of angular position, angle of rotation or phase. As shown in FIGS. 7 and 8, the member for adjusting an angular position, angle of rotation or phase (which is configured by an attachment member) requires a length to some extent to accurately represent its angular position, angle of rotation or phase. Therefore, a total height of the rotary disc is increased by that extent, and the axial length of the encoder is increased accordingly. This is a problem.
If a rotary disc consists of two members, the cost is elevated consequently. Further, an error associated with assembling of the two members lowers the phase adjusting accuracy. Therefore, it is envisioned that the disc forming a signal generation section and the attachment member having a phase-adjustment portion for use in attachment to the rotary shaft are integrated and resin-molded to solve such the problem. As the two members have quite different shapes, however, it is difficult to find a molding condition to mold them integrally.
If the rotary disc is automatically assembled to the rotor shaft of the motor, a robot is employed to handle the rotary disc. As shown in FIG. 7, a grip like the non-rotational symmetric section 18 is required.